1. Field of the Invention
The present invention is directed to tarnish-resistant and adhesion promoting treatment compositions, and more particularly to tarnish-resistant and adhesion promoting treatment compositions that are substantially free of chromium.
2. Brief Description of the Art
Tarnish resistance of metal and tenacious adhesion between metals and polymer has been long sought for various applications such as printed circuits, electronic packaging, automotive body trim (aluminum to PVC or ABS plastics, for example), steel belted tires, and paint or other polymeric coatings. For example, the tarnish resistance of Cu foil for printed circuit application is required for shelf life and to withstand the heat exposure during lamination and soldering in printed circuit processing. The antitarnish coatings also prevent the oxidation of Cu and reaction with the polymeric substrate upon humidity and temperature exposures during the service life of the parts. This function is critical to maintain a good bond between Cu and polymer. In terms of electronic packaging applications, good adhesion is required in an electrical connector between a metallic jack and the molded plastic insulating sheath that surrounds it. As a second example, the inner leads of a metallic leadframe may be encapsulated in a polymer molding resin in the assembly of an electronic package to house an integrated circuit.
The adhesion between a polymer resin and a metallic substrate is improved by coating the metallic substrate with a second metal that forms a more tenacious bond to the polymer resin. Suitable coatings include refractory oxide-forming metals such as nickel and nickel alloys, or intermetallic-forming coatings such as tin on a copper substrate. These coatings, as well as others, are disclosed in U.S. Pat. No. 4,888,449 to Crane et al., which is incorporated by reference in its entirety herein.
Currently, zinc/chromium (ZnCr)-containing coatings are widely used on copper foils and lead frames in electronic applications to improve the tarnish resistance and bond strength of metal to polymeric substrates. These coatings are typically electrodeposited in an alkaline solution containing NaOH, Zn and Cr(VI) ions to form a mixture of Zn/Cr oxides, which provide tarnish resistance.
U.S. Pat. No. 4,387,006 discloses a method of treating the surface of a copper foil used in printed wire boards. The copper foil is electrolytically treated in an aqueous solution of zinc chromate containing zinc ions and chromium (VI) ions and immersed in an aqueous solution containing aminosilane, sodium silicate, and/or potassium silicate.
U.S. Pat. Nos. 5,230,932; 5,098,796; and 5,022,968 disclose a technique and compositions for improving the tarnish and oxidation resistance of copper and copper-based alloy materials. The material is electrolytically coated by immersion in an aqueous electrolyte containing NaOH, zinc ions and chromium (VI) ions. The coating provides tarnish resistance at temperatures in excess of 190° C.
U.S. Pat. No. 5,250,363 to Chen discloses a technique for improving the tarnish and oxidation resistance of a metallic substrate. The substrate is immersed in an aqueous solution containing NaOH, zinc ions, and chromium (VI) ions, and a chromium-zinc coating is electrolytically applied to the substrate. The coating provides tarnish resistance at temperatures in excess of 230° C., and is easily removable by immersion in sulfuric acid.
It is also claimed that under certain conditions, several of these coatings could reduce moisture penetration between lead frame and molding compound. This effect could greatly reduce the “pop corn effect,” which is believed to be caused by formation of water vapor from accumulated moisture trapped under the die attach paddle when the device is subjected to a soldering temperature (e.g., temperatures of approximately 230° C.). This generation of water vapor is believed to be the cause of device failure due to cracks and delamination. Moreover, as industry moves away from solder containing lead and other toxic elements due largely to environmental concerns and tightening regulations, alternative solders are likely to have even higher soldering temperatures, and the attributes of metal/polymer adhesion will likely become more important.
Many compositions are known that provide for adhesion between a metal and a polymeric substance. U.S. Pat. No. 5,573,845 to Parthasarathi et al. discloses a composite material having a metallic core with a desired surface finish. An acicular (nodular) superficial coating layer having an apparent thickness of less than 275 angstroms is adjacent to at least a portion of the metallic core. The superficial coating layer is removable from the metallic core without appreciable change to the metallic core surface finish.
U.S. Pat. No. 5,449,951 to Parthasarathi et al. discloses a lead frame with enhanced adhesion to a polymer resin. The lead frame is coated with a thin layer containing chromium, zinc, or a mixture of chromium and zinc. The coated leadframe exhibits improved adhesion to a polymeric resin.
In International Publication Number WO 00/74131 A1 to Lee et al. a method of assembling a semiconductor device package is disclosed wherein a device/leadframe assembly is formed and then an adhesion enhancing coating is deposited on the exposed surfaces of the device/leadframe assembly, followed by encapsulation. The coating is a metallic coating which could be an inorganic Zn—Cr coating. This publication is specifically incorporated by reference herein in its entirety.
U.S. Pat. No. 5,252,855 to Ogawa et al discloses improving the adhesion between a copper base metallic substrate and a polymer resin by anodically oxidizing the substrate to form an aggregate of acicular crystals. Anodization of aluminum alloy substrates enhances the strength of a polymer bond as disclosed in U.S. Pat. No. 4,939,316 to Mahulikar et al. Other satisfactory coatings include oxidation resistant materials having a thickness of 300-5,000 angstroms as disclosed in U.S. Pat. No. 5,192,995 to Yamazaki et al.
As disclosed in the above art, sodium (Na) and/or potassium (K) ions are frequently used in immersion baths. However, sodium and potassium ions suffer a disadvantage in that these ions could potentially diffuse into the silicon die and cause failures in the produced devices. Such failures have been well documented (See, for example “Metallization Contamination,” Microelectronic Defects Database, CALCE Electronic Products and Systems Center, University of Maryland, Apr. 12, 2000, identifying chloride and sodium contamination from molding compounds, die coatings, and die attach coatings; and Barnes and Robinson, “The Impact of Ionic Impurities in Die Attach Adhesives on Device Performance”, Proceedings of 34th Electronics Components Conf., May, 1984, p. 68, identifying die attach adhesives as a particular source of ionic contamination).
In addition, the presence of Cr(VI) ions in a plating solution invokes environmental concerns in handling and disposal. Although Cr(VI)-containing plating baths have been used throughout the world for decades, the use of Cr(VI) is coming under increasing scrutiny due to growing concerns with health and environmental effects. Thus, it would be desirable to either replace a Cr(VI) plating bath with a less toxic Cr(III) bath, or eliminate the Cr entirely.
Thus there is a need in the art to provide aqueous adhesion promoting and tarnish resistance treatment compositions that are substantially free of chromium (VI) and that utilize optional electrolytes that do not contain potassium or sodium ions. The present invention is believed to be an answer to that need.